The present invention relates to apparatus and methods for separating and recovering valuable raw materials. It particularly relates to processes for recovering polymer such as polyester and polycarbonate plastics from multi-layer sheets and films where such polymer has a thin coating of saran, metal, and/or other substance thereon to improve the gas permeation resistance, toughness, or other characteristics or to provide an emulsion coating as in photographic film.
My prior invention, entitled "Polymer Layer Recovery from a Multi-Layered Chip Material", U.S. Pat. No. 4,629,780, discloses and claims wet chemical processes for separating saran and photographic emulsion layers from polyester and polyvinylchloride layers in photographic and beverage container sheets and films. The wet process has some advantages, including recovery of silver as disclosed in my prior U.S. Pat. entitled "Filter Process for Silver Recovery from Polymeric Films", U.S. Pat. No. 4,765,835. Polycarbonate bottles particularly require recycling to make such use of the expensive plastic economically feasible.
Dry processes for recycling raw materials from plastic films are known in my invention in "A Dry Process for Polymer Recovery", U.S. application Ser. No. 020,470, filed Mar. 2, 1987. Chopping of photographic film and bottles into chip size materials to afford easier handling and to enable separation of metal caps, paper label-bearing portions, and polyethylene bottoms is known. My U.S. Pat. No. 4,775,697 provides a dry abrasive process usable in abrading of chips of a multi-layered film to remove coating layers to recover pure polymer. However, chips may have up to 20% liquid (water, sodium hypochlorite, etc.) on them from silver-removal and other processes. In unrelated arts, ball or rolling mills are used to abrade and polish decorative stones. Rotating mills are used for mixing components such as in fertilizer, and for coating aggregate with asphalt for paving operations. Such ball or rolling mill and other rotating processes have not, to my knowledge, ever been used by others to abrade plastic for recycling.
In summary the present invention comprises a hot abrasive method of drying and then separating and isolating a substantially pure polymer such as a polyester or polycarbonate from a wetted, multi-layer film or sheet having a body of pure polymer in one layer and having one or more other substances such as saran or cellulose triacetate in one or more other, thin, coating layers. Photographic film and plastic drink and beverage containers are primary examples of such films and sheets. The method comprises a first step of placing one volume of chips of the film or sheet in a rotating mill, pug mill, or the like together with about three volumes of small, hard, abrasive particles such as quartz, quartzite rock, glass cullet, or igneous rock of about one sixty-fourth to one-half inch size. The aggregate has been heated to 165.degree. F. to 300.degree. F.
The mill is rotated to mix the hot aggregate with the chips, to dry the chips and to collide and abrade the surfaces of the chips with the abrasive particles, for a selected time in a batch process or for a selected dwell time in a continuous process. The time is generally less than five minutes but is experimentally determined; it varies with the wetness of the chips (up to 20%), the nature of the coating of the polymer, the particular aggregate particles used, and with other factors such as the size of the mill, the fill ratio, the dryness of the chips and aggregate materials used. A second rotating mill, pug mill, or the like can be used in line with the first, with about two volumes of additional hot aggregate added to maintain the temperature needed for complete drying and separation; this second milling is not always needed.
Successful treatment and purity of the resulting polymer is readily determined by a melting test. The aggregate particles, the pure sheet or film remnants, and the coating dust are separated from one another by air classifiers, and the materials are then conveyed to collection and reuse locations. Polymer dust may also be recovered by a further separation step .